A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or antiparallel to this direction.
In the lithographic apparatus, various parts are fastened to one another using fasteners. It is known, for example, to use bolt, screw or stud fasteners for fastening apparatus parts together. For example, a first apparatus part, such as an optical component, may be clamped onto a second apparatus part, such as a base or a support frame, using one or more fasteners.
The known fastener, e.g. a bolt, screw, or stud, includes a shaft that has screw thread of a certain pitch, as well as a driving head for driving the fastener into a suitable aperture of a respective apparatus part. After assembly, the fastener clamps the apparatus parts together, using cooperation of the screw thread of the shaft and the screw aperture, and the driving head.
Generally, it is desired to provide a relatively firm clamp so that the apparatus parts can be connected to each other firmly and durably. It is known to provide such a firm clamp by using a certain spring tension. This spring tension can be provided by a certain elasticity of the apparatus parts which are to be clamped together. However, in many cases, the apparatus parts may not be suitable to provide or sustain such elasticity. Also, in various cases, deformation of one or more of the apparatus parts may not be desired, for example, because such deformation may result in misalignment of the apparatus part.
The material of each fastener as such may have a certain elasticity which can result in a certain spring tension after the fastener has been mounted. For example, the free length of the screw may be allowed to stretch for providing a certain clamping tension. However, in many cases, a certain attainable free length of the screw (or bolt or stud), a certain desired clamping force, a certain allowable torque, a certain relaxation reserve and/or a desired diameter cannot be designed or selected satisfyingly. For example, in certain cases, the free length of the fastener has to be so large for providing sufficient tension after assembly that there is insufficient space left in the apparatus for mounting the fastener.
Moreover, it is known to use a separate spring or spring washer, which is placed between the head of the fastener and an apparatus part, for securing a certain clamping tension after assembly. However, the application of such separate, usually relatively small, spring parts or washers is not desired for production and service, because such part or washers can easily be dropped and lost in the apparatus during assembly. Small spring parts or washers are relatively difficult to handle and may therefore lead to assembly difficulties or delays.
If the fastener is to be used in or in contact with a vacuum environment, it is desirable mount the fastener in a way to prevent dead volumes, where residual gasses can be trapped for a certain period of time. Such dead volumes may hamper the pumping down of the apparatus. Also, gasses escaping from such dead volumes may contaminate the desired vacuum. Special holes may be drilled in apparatus parts to prevent or remove such dead volumes. However, drilling holes in apparatus parts is costly and may negatively affect the functioning of the apparatus parts. An axial hole may be provided in the stud, screw or bolt fastener for opening any dead volume which may lie axially below the fastener. However such axial holes can not reach all dead volumes which may be trapped by the fastener after assembly.
The disadvantages of known fasteners and fastening methods of apparatus parts may reduce overall apparatus performance, lessen throughput and decrease the operating accuracy of the apparatus. This leads to higher costs and/or lower performance of devices, which are made by the apparatus using a lithographic manufacturing method.